Power supply lines in an integrated circuit (IC) can supply current to charge and discharge active and passive devices in the IC. For example, digital complementary metal-oxide-semiconductor (CMOS) circuits draw current when the clock makes a transition. During operation of circuits, the power supply lines supply transient currents with a relatively high intensity, which can result in voltage noise in the power supply lines. The voltage in the power supply line will fluctuate when the fluctuation time of the transient current is short or when its parasitic inductance or parasitic resistance is large.
The operational frequency of the IC may be on the order of several hundreds of mega-hertz (MHz) to several giga-hertz (GHz). In such circuits, the rising time of clock signals is very short, and voltage fluctuations in the supply line may be very large. Undesired voltage fluctuations in the power supply line powering a circuit can cause noise on its internal signals and degrade noise margins. The degradation of noise margins can reduce circuit reliability or even cause circuit malfunction.
To reduce the magnitude of voltage fluctuations in the power supply lines, filtering or de-coupling capacitors are usually used between the different power supply lines or between the power supply line and the ground line. De-coupling capacitors act as charge reservoirs that additionally supply currents to circuits to prevent momentary drops in supply voltage.